Not applicable.
Not applicable.
In the process of making semiconductor chips, the exhaust streams of some tools may contain F2. For example, a new tool from Applied Materials uses microwave to dissociate NF3 to clean a reactor chamber. Although the exhaust stream contains only traces of NF3, significant quantities of F2 exist. Streams, such as the exhaust from the Applied Materials tool, have to be treated to remove F2 before venting to the atmosphere. Fluorine is a corrosive and toxic chemical with a TLV of 1 ppm.
Conventionally, F2 is removed using wet caustic scrubbers where F2 reacts with the caustic solution to form fluoride salt. The wet scrubber, although efficient, produces large quantities of liquid wastes that need to be treated later. In addition, the caustic scrubbers can generates toxic byproducts, OF2 and NO3F, that causes additional safety concerns.
Alternatively, F2 has been removed using solids. A packed bed of charcoal has been used to remove F2, but it generates global warming carbon-fluorine gases. Safety is another concern in using carbon, as the possibility exists of forming explosive CFx compounds in the packed bed. Packed beds of soda lime (NaOH/Ca(OH)2), limestone (CaCO3), and alumina (Al2O3) have been employed to remove F2. These dry scrubbers frequently plug and require considerable manpower to empty and refill the reactors. In addition, the heat generation inside a packed bed could present a problem when F2 concentration in the feed becomes too high, as the heat removal inside a packed bed is a challenge. Other methods of F2 disposal can be found in the report by Netzer, W. D., Fluorine Disposal Processes for Nuclear Applications, Goodyear Atomic Corp., Apr. 8, 1977.
Holmes, et. al., Fluidized Bed Disposal of Fluorine, I and EC Process Design and Development, Vol. 6, No. 4, October 1967, pp. 408-413, describes the abatement of fluorine using a fluidized bed of activated alumina. High F2 removal efficiency ( greater than 99%) was easily achieved at a reaction temperature between 300 to 400xc2x0 C. The flow rate was limited to 1.25 to 1.65 minimum fluidization velocity. The definition of minimum fluidization velocity can be found in many textbooks, e.g. xe2x80x9cFluidization Engineeringxe2x80x9d by Kunii and Levenspiel, John Wiley and Sons, 1969. In this velocity range, the fluidized bed is considered at a xe2x80x9cbubblingxe2x80x9d fluidization mode. Soda ash was also found to be effective.
U.S. Pat. No. 5,417,948 discloses the use of zirconium alloys to abate NF3. It lists fluidized beds as a possible means of contacting the alloys with NF3. A control example used iron wire cut into 5 to 10 mm pieces as a bed material.
The prior art has attempted to provide various methods and means of abating fluorine. However, the prior art has not achieved a commercially viable process for fluorine abatement which generates no pollutants, allows for high throughput, avoidance of clogging, efficient destruction of fluorine and a method for recharging of the abatement system for continuous processing. These advantages are achieved by the present invention, as will be set forth in greater detail below.
The present invention is a process of destroying a fluorine specie selected from the group consisting of fluorine, chlorine trifluoride and mixtures thereof from a gas mixture containing a fluorine specie by contacting the gas with a fluidized bed of metal particles capable of reacting with a fluorine specie wherein the particles have a particle size essentially no greater than approximately 300 microns.
Preferably, up to 10 wt. % of the bed of metal particles are large particles having a particle size sufficiently larger than 300 microns to assist in mixing of the bed of metal particles.
Preferably, the large particles have a particle size of approximately 500 to 2000 microns.
Preferably, the metal particles are selected from the group consisting of iron, nickel, copper, calcium, aluminum, magnesium, manganese, cobalt, zinc, tin and mixtures thereof.
Preferably, the contacting is performed at a temperature in the range of 150 to 550xc2x0 C.
Preferably, the gas is contacted with the fluidized bed until the height of the fluidized bed increases due to reaction with the fluorine to a predetermined increased bed height.
Preferably, the gas alternately contacts one of at least two parallel switching fluidized beds where one bed is contacting the gas while one or more other fluidized beds are being recharged.
Preferably, the gas contact is switched from a first of at least two parallel switching fluidized beds to an other fluidized bed that has been recharged when the first bed has expanded to at least approximately 90 percent of the original bed height.
Preferably, after the gas contacts the fluidized bed the gas contains no greater than 1 part per million by volume of fluorine specie.
Preferably, the flow of the gas is sufficient to obtain at least a minimum fluidization velocity of the fluidized bed for the metal particles contained in the fluidized bed.
Preferably, the gas has a residence time, defined by the ratio of packed bed volume to the volumetric feed flowrate at normal conditions, in the fluidized bed of greater than approximately 3 seconds.
Preferably, the flow of the gas is approximately at least two times the minimum fluidization velocity.
Preferably, the gas contains a gas component selected from the group consisting essentially of N2, O2, Ar, He, SiF4, NF3, CF4, C2F6, CHF3, SF6 and mixtures thereof.
Most preferably, the present invention is a process of destroying fluorine in a gas containing fluorine and one or more gas components of N2, O2, Ar, He, SiF4, NF3, CF4, C2F6, CHF3, SF6, and mixtures thereof by contacting said gas alternately with one of a pair of switching parallel connected fluidized beds of iron metal particles capable of reacting with fluorine wherein the particles have a particle size essentially no greater than approximately 300 microns, and switching beds when the height of the bed being contacted with said gas increases to a predetermined bed height corresponding to substantially stoichiometric reaction of fluorine with the iron metal particles.
Preferably, the iron metal particles are at least approximately 99% iron by weight.
Preferably, the iron metal particles have an average particle size of approximately 100 microns.
Preferably, the iron metal particles react with the fluorine to generate a mixture of FeF2 and FeF3.
In one preferred embodiment, the gas containing fluorine contains predominantly nitrogen and such fluorine.
In an alternate embodiment, the metal particles are continuously being added to the fluidized bed and metal particles that have been reacted with the fluorine are continuously being removed. In this alternate embodiment, preferably a single fluidized bed is utilized.
Not Applicable
The present invention is a method of destroying a fluorine specie of fluorine and/or chlorine trifluoride using fine metal powder in a fluidized bed reactor. The preferred metal powder is iron and should have a purity of greater than 90%, preferably 99% by weight and a particle size essentially no greater than approximately 300 microns, preferably an average particle size of approximately 100 microns. Preferably, up to 10 wt. % of the particles in the bed can be larger than 300 microns with sufficient size to assist in mixing of the particles of essentially no greater than approximately 300 microns during operation and fluidization to prevent caking of the latter particles. These larger particles could preferably have a size of 500 to 2000 microns.
The preferred operating conditions are:
temperature; 150xc2x0 C. to 550xc2x0 C.;
feed flow; greater than the minimum fluidization velocity of the powder;
residence time; greater than 3 seconds.
Other metals such as Ni, Cu, Ca, Al, Mg, Mn, Co, Zn and Sn that can form nonvolatile metal fluorides will also be good candidates. The reaction conditions, however, will vary depending on the metal of choice.
The objective of this invention is to provide a cost-effective method of destroying fluorine, F2. Fluorine is a toxic and corrosive chemical with a threshhold limit value (TLV) of 1 ppm. Wet scrubbing with a caustic solution is not effective. Dry scrubbing using packed beds of soda ash, limestone, or alumina has bed clogging problems. The present abatement method provides a simple alternative to fluorine abatement techniques of the prior art, while producing no hazardous waste.
The present invention suggests a method of destroying fluorine in a fluidized bed of preferably fine iron powder. More specifically, a powder with high purity ( greater than 99 wt. %) prepared, for example, electrolytically with a particle size of essentially no greater than approximately 300 microns can be used for this application. The high purity iron powder, which is commercially available at a reasonable cost, minimizes the generation of other impurities. The small particle size provides adequate fluidization of the iron-bed and rate of reaction between the iron and the fluorine specie.
The presumed reactions between Fe and F2 are:
2Fe+3F2xe2x86x922FeF3xe2x80x83xe2x80x83(1)
Fe+F2xe2x86x92FeF2xe2x80x83xe2x80x83(2)
Theoretically, each mole of Fe (56 grams) is capable of removing 38 grams or 57 grams of F2 depending on whether the products are FeF2 or FeF3. No NOx is formed. Other metals, such as Ni, Cu, Ca, Al, Mg, Mn, Co, Zn and Sn and their alloys that can form nonvolatile metal fluorides, will also be good candidates. The reaction conditions, however, will vary depending on the metal of choice.
Operating conditions will depend on feed composition and flowrate. The fresh iron powder can start to react with fluorine at a temperature as low as 150xc2x0 C. The method works practically under any operating pressure. Conveniently, an exit pressure close to atmospheric or under slight vacuum will be adequate. Depending on the feed concentration, operating pressure and residence time, the reactor temperature can be chosen to remove fluorine completely. The velocity flowing through the reactor should be high enough to fluidize the solid powder. The fluidization velocity can be measured by plotting pressure drop across the bed versus flow as described in xe2x80x9cFluidization Engineeringxe2x80x9d by Kunii and Levenspiel and published by John Wiley in 1969. When a significant drop-off in the rate of pressure drop increase for increasing flow of fluidization gas is measured, fluidization has occurred. Fluidization as used herein includes percolation or ebulation as those terms are understood in the art. The fluidization should be sufficient to elevate the particles initially without having them removed from the bed initially so that an internal circulation of the particle is created with the particles rising and falling due to proximity to the fluidizing gas. The bed of particles that are being fluidized would take on the properties of a fluid, or near fluid if they are merely being percolated. Preferred fluidization rates are from 2 to 10 times the minimum fluidization rate for the metal particles. Continuous operation could be possible with the fluidized bed reactor design by utilizing appropriate solid addition and withdrawal hardware.